I guess that depends...I think the source MAC header of the wireless client is preserved when it traverses the AP and is only rewritten at a router.
In my opinion, the best approach would be for the UTM to get aggregate a list of the MACs of connected wireless clients across all APs, and then push that as an ACL back to all APs so that all traffic between clients is dropped at L2.
I am no developer so maybe this isn't a feasible approach -just the way I see it in my network engineering "mechanics" brain [;)]